Effect of biomass particles on biomass boilers

2019-11-04

Biomass pellet fuel characteristics
The fuel characteristics mainly include: water mass fraction, volatile mass fraction, elemental composition, calorific value, ash mass fraction and its composition, particle size distribution, fracture characteristics, density and so on. The particularity of biomass pellet fuel is mainly reflected in the following aspects:

  1. The biomass pellet fuel has a high volatile content, a low precipitation temperature, and a short precipitation process.
  2. The water quality fraction is variable, and high moisture has a great influence on the initial stage of combustion.
  3. After the biomass is burned, the ash is small, and the texture of the ash is soft.
  4. Biomass pellet fuel has high semi-coke activity and burns as quickly as possible.
  5. The biomass pellet fuel has a low sulfur mass fraction and a low concentration of pollutant emissions.
  6. Most annual herbaceous biomass has high potassium and chlorine content.

The effect of biomass pellet fuel characteristics on the biomass boiler
The influence of fuel characteristics on the boiler is mainly reflected in the difficulty of fire, the stability of combustion, the slagging and bonding, the characteristics of ash formation and the mass concentration distribution of ash in the furnace, combustion efficiency, pollutant discharge, heating surface layout, and crushing. Selection of the system and selection of auxiliary machines.
Biomass biomass combustion process: first, the fuel particles are heated and dried by the high temperature bed material; then the fuel pyrolysis and volatile combustion; then some particles will expand and break first; and finally the coke burns with wear and tear phenomenon.

Biomass pellet fuel is particularly flammable, and volatile combustion is completed in a short time (about 3 s). The main stage of semi-coke combustion is 8 to 10 s after the fuel is put into the furnace, and the basic burning of the semi-coke takes 45 to 50 s. Therefore, the biomass is significantly different in the combustion process of the furnace and the combustion process of the coal in terms of time of occurrence and combustion history.

Influence of ash mass fraction
Biomass requires a large amount of bed material particles to circulate in the circulation loop, resulting in a more even heat distribution, faster heat transfer, and more complete combustion.
1. Effect on combustion
The increase in ash mass fraction increases the boiler material circulation and reduces the furnace combustion temperature, especially for biomass boilers with medium temperature separation. The biomass biomass boiler with medium temperature separation generally has a return temperature of about 620 ° C. When the ash content is greatly increased, the return temperature is lower than 550 ° C, so that the furnace temperature is rapidly lowered, sometimes even causing Furnace fire extinguishing accident.

2.Impact on load
The ash mass concentration in the flue gas has a great influence on the heat transfer coefficient of the biomass boiler furnace. When the ash mass concentration increases, the convection and radiation heat transfer coefficient of the water wall will increase. Therefore, the ash mass concentration is used as the adjustment load and bed temperature. The means are widely adopted.

3. Effect on the material layer
It is important to maintain the relative stability of the bed height during operation of the biomass boiler. If the bed is too high or too low, it will affect the fluidization quality and cause coking.

4. Influence on the arrangement of the heating surface of the tail
The fuel ash mass fraction affects the mass concentration of fly ash in the flue gas, thereby affecting the ash and wear of the heated surface of the tail. Due to the influence of the amount of ash in the biomass pellet fuel, the ash accumulation in the tail economizer is severe, so the structure of the economizer is changed from the original staggered arrangement to the in-line arrangement.

In order to minimize the adverse effects of changes in the ash mass fraction in the fuel, when the fuel contains a lot of ash, the bottom slag is generally adopted. When the biomass pellet fuel contains less ash, the amount of ash on its own cannot meet the requirements of bed material circulation, and it is generally necessary to add bed material in operation. The extra bed material becomes the main body of the circulating material, and should be not easily worn, such as quartz sand, river sand, and the like.

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